ADP3207 ANALOG DEVICES, ADP3207 Datasheet - Page 24

• Manufacturer Part Number: ADP3207
• Description: 7-Bit Programmable Multiphase Mobile CPU Synchronous Buck Controller
• Manufacturer: ANALOG DEVICES

ADP3207
where:
n
R
0.5 Ω for two pieces of typical high speed switching MOSFETs,
making R
C
thing to reduce switching loss is to use lower gate capacitance
devices.
The conduction loss of the main MOSFET is given by
where:
R
Typically, for main MOSFETs, users want the highest speed (low
C
must select a device that meets the total power dissipation
(0.6 W for a single 8-lead SOIC package) when combining the
switching and conduction losses.
For example, using an IRF7821 device as the main MOSFET
(four in total; that is, n
and R
as the synchronous MOSFET (four in total; that is, n
R
dissipation per MOSFET at I
420 mW for each synchronous MOSFET and 410 mW for each
main MOSFET.
One last consideration is the power dissipation in the driver for
each phase. This is best described in terms of the QG for the
MOSFETs and is given by the following equation:
where:
Q
Q
Also shown is the standby dissipation (I
For the ADP3419, the maximum dissipation should be less than
300 mW, considering its thermal impedance is 220°C/W and
the maximum temperature increase is 50°C. For this example,
with I
mW dissipation in each driver, which is below the 300 mW
dissipation limit. See the ADP3419 data sheet for more details.
MF
G
DS(MF)
ISS
ISS
DS(SF)
GMF
GSF
is the total gate resistance (1.5 Ω for the ADP3419 and about
) device, but these usually have higher on-resistance. Users
is the total number of main MOSFETs.
is the input capacitance of the main MOSFET. The best
is the total gate charge for each synchronous MOSFET.
P
P
is the total gate charge for each main MOSFET.
DS(MF)
CC
C
DRV
= 6.7 mΩ (max at T
is the on-resistance of the MOSFET.
(
MF
= 2 mA, Q
G
)
=
= 2 Ω).
= 18 mΩ (max at T
=
⎡
⎢ ⎣
D
2
f
SW
×
×
n
⎡
⎢
⎢
⎣
⎛
⎜
⎜
⎝
×
n
GMF
(
I
MF
n
O
MF
MF
= 14 nC and Q
⎞
⎟
⎟
⎠
×
2
= 4), with about C
+
Q
J
GMF
12
= 120°C). Solving for the power
1
O
×
= 32 A and I
J
+
⎛ ×
⎜
⎜
⎝
= 120°C) and an IR7832 device
n
n
n
SF
MF
I
×
R
Q
GSF
⎞
⎟
⎟
⎠
GSF
2
CC
⎤
⎥
⎥
⎦
= 51 nC, there is 120
×
)
× V
+
R
R
ISS
DS
I
= 10.7 A yields
CC
(
= 1010 pF (max)
CC
MF
⎤
⎥ ⎦
) of the driver.
×
)
V
CC
SF
= 4),
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RAMP RESISTOR SELECTION
The ramp resistor (R
PWM ramp. The value of this resistor is chosen to provide the
best combination of thermal balance, stability, and transient
response. Use this equation to determine a starting value
where:
A
A
R
C
Another consideration in the selection of R
internal ramp voltage (see Equation 22). For stability and noise
immunity, keep this ramp size larger than 0.5 V. Taking this into
consideration, the value of R
The internal ramp voltage magnitude can be calculated using:
The size of the internal ramp can be made larger or smaller. If it
is made larger, then stability and transient response improves,
but thermal balance degrades. Likewise, if the ramp is made
smaller, then thermal balance improves at the sacrifice of
transient response and stability. The factor of three in the
denominator of Equation 21 sets a minimum ramp size that
gives an optimal balance for good stability, transient response,
and thermal balance.
COMP Pin Ramp
There is a ramp signal on the COMP pin due to the droop
voltage and output voltage ramps. This ramp amplitude adds to
the internal ramp to produce the following overall ramp signal
at the PWM input:
For this example, the overall ramp signal is found to be 1.5 V.
DS
R
D
R
is the internal ramp amplifier gain.
is the internal ramp capacitor value.
is the current balancing amplifier gain.
is the total low-side MOSFET ON-resistance,
V
V
V
R
R
R
R
R
R
RT
=
=
=
=
=
3
3
0
A
280
⎛
⎜
⎜
⎝
2 .
×
×
R
1
R
5
A
×
×
−
0
R
k
×
D
2 .
1 (
A
1 (
n
×
Ω
3
×
R
×
×
C
−
4 .
−
×
2
×
R
360
R
. 0
f
×
m
D
DS
5
SW
L
×
061
(
)
pF
V
Ω
R
1
×
) is used for setting the size of the internal
nH
×
f
R
×
−
×
C
SW
V
×
)
n
C
R
5
×
VID
280
×
X
pF
. 1
D
×
150
R
kHz
)
R
=
is selected as 280 kΩ.
O
282
V
⎞
⎟
⎟
⎠
=
k
Ω
. 0
55
V
R
is the size of the
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